Yeah and now thing about it!
When a mighty warrior creates a soul crystal, he put his experience and knowledge inside a gem and we call it a soul crystal! He created a soul gem by splitting a part of his own soul in that object. That is a form of creation magic and also can distinguish the soul crystals of people, because they imbue it with there own soul.
That is the same concept, if a ancient create something out of his own existence he split up a part of his soul to create something new. That is why Emeth can distinguish it, because it is produced by inserting aether on a concept that contains a part of the soul of a ancient.
This concept is valid because we know from the ancient, that when they want to die, they unmake themself by entering the aetherial sea. And other ancient are born out of that aetherial sea, like everything that has a soul is later also born out of the same spitted version of the aetherial sea. That is the whole concept of mostly any Final Fantasy same, mako energy in ff14 with the soul stream and so on.
And that ancients are pure soul is also valid, because some of the ascians are ancients, like emeth and he needs a body to put his soul in. Also we know from Aulus mal Asina, the scientist in stormblood, that you can cut the soul out of the body and insert them inside other beings. Like in the endwalker quest where we get cut out of the body and inserted in a body of a normal soldier.
The only conclusion we can get out of that is, that soul is aether. And if you build something out of aether that is alive you build also a soul inside of it. The only thing that is valid is, that ancient can't build is "emotions" aka dynamis. Because they never mentioned the concept, that beings with souls produce another form of energy. The only exception of that was a being produced with more dynamis as aether. And that lead to an overflow of emotions and doomed themself. Because it was to to feel all the emotions inside the existence of the universe.